(643g) Towards the Renewable Production of Isobutanol from Sustainable Chemicals

The growing demand for sustainable fuels and chemical intermediates has driven interest in chemicals such as isobutanol. Its production can be carried out using renewable sources. On the one hand, microbial fermentation constitutes a promising method due to its environmental and cost advantages, although challenges remain in improving fermentation titers and developing efficient separation techniques (Su et al., 2020). Isobutanol’s recovery is highly enhanced through methods such as vacuum distillation, gas stripping, and alternative separation techniques to reduce energy consumption (Fu et al., 2021; Janković et al., 2024). On the other hand, isobutanol can be synthesized from methanol and ethanol, leveraging their increasing production from renewable sources. However, this process also requires efficient purification and recycling strategies to ensure economic and environmental feasibility.

In this work, a superstructure comprising five different process configurations is proposed to produce synthetic isobutanol. Each configuration incorporates unit operations to enable the conversion of methanol and ethanol into isobutanol while ensuring the effective recovery and recycle of unreacted feedstocks. The objective is to identify the most efficient process alternative by evaluating different technological routes based on economic, energy, and environmental performance indicators. The five proposed processes for isobutanol production are designed to include different recovery techniques for methanol, ethanol, and purification of isobutanol. The first process uses a distillation column to recover almost all methanol and ethanol, which are recycled to the reaction section, while isobutanol and water are separated in an extraction column employing pentane. The second process also employs distillation for methanol and ethanol recovery but uses pressure swing adsorption (PSA) for isobutanol purification. The third process is like the second but uses pervaporation instead of PSA. In the fourth process, PSA is applied after the reactor to remove water, and the remaining mixture is distilled to recover isobutanol with a purity of nearly 99%. Any unreacted methanol and ethanol are subsequently recycled to the reaction section. The final process employs an extraction column with pentane, followed by two distillation columns: one for water removal and methanol/ethanol recirculation, and another to recover nearly pure isobutanol, with a recycle composed of pentane. Each of these processes is modeled in detail using an equation-based approach, using experimental data for the reactor operation (Wingad et al., 2016), along with thermodynamic equilibria and phase behavior. Additionally, mass and energy balances, thermodynamic principles, chemical equilibria, and both experimental and theoretical yields are considered. Surrogate models obtained through sensitivity analysis based on rigorous simulations are used to determine the compositions and temperatures of the streams in the distillation and extraction columns, as well as the condenser and reboiler duties of the distillation column. A techno-economic analysis is performed to assess the investment and operational costs, and the isobutanol yield and purity. The key performance indicators (KPIs) used to evaluate the processes include energy consumption, carbon footprint, investment requirements, and product purity. The recycling of methanol and ethanol is a critical aspect of the study, as it directly impacts the overall efficiency and sustainability of the production process.

To determine the optimal process configuration, five different nonlinear programming (NLP) models are formulated, incorporating economic, environmental, and operational constraints. The model aims to minimize the total production cost while maximizing isobutanol yield and purity and ensuring compliance with process feasibility criteria. The optimization problem is solved using a multi-objective approach to balance trade-offs between investment, energy consumption, and environmental impact. In addition to process selection, a sensitivity analysis is conducted to evaluate the impact of methanol and ethanol price variations on the economic viability of isobutanol production. The price fluctuations of methanol and ethanol are considered based on their different production pathways, including conventional fossil-derived routes and renewable synthesis methods. Renewable methanol, produced from biogas (Hernández and Martín 2016), biomass gasification (Martín and Grossmann, 2017), through carbon capture and hydrogenation processes (Galán et al., 2023), and bioethanol, obtained from syngas (Martín and Grossmann, 2011), sugars fermentation (Martín and Grossmann, 2012), exhibit different cost structures and market dynamics compared to their fossil-based counterparts. The sensitivity analysis explores various pricing scenarios to determine the economic resilience of the selected process under different market conditions.

The evaluation of the five process configurations provides insights into their potential feasibility for synthetic isobutanol production. The integration of reaction and separation operations plays a crucial role in optimizing energy efficiency, maximizing isobutanol purity and yield, and reducing product cost. Additionally, the assessment of methanol and ethanol recovery highlights the importance of efficient recycle strategies in reducing raw material costs and enhancing process sustainability. Notably, the second and third configurations based on the use of PSA and pervaporation emerge as the most promising alternatives due to their greater simplicity, as they require less equipment and fewer utilities. Nevertheless, more detailed analyses are necessary to fully assess their performance and feasibility. Furthermore, the sensitivity analysis on methanol and ethanol prices underscores the economic implications of sourcing these feedstocks from renewable versus fossil-derived pathways. On the one hand, renewable methanol and bioethanol contribute to lower carbon footprints, but their higher production costs could impact the economic competitiveness of isobutanol synthesis. On the other hand, fossil-based methanol and ethanol offer cost advantages, although they may not align with sustainability goals.

Acknowledgments

The authors acknowledge the funding received from Ministerio de Ciencia Innovacion y Universidades, PID2023-1462310B-I00.

References:

Fu, C., Li, Z., Jia, C., Zhang, W., Zhang, Y., Yi, C., Xie, S. (2021). Recent advances on bio-based isobutanol separation. Energy Conversion and Management: X, 10, 100059. https://doi.org/10.1016/j.ecmx.2020.100059.

Galán, G., Martín, M., Grossmann, I. E. (2023). Systematic comparison of natural and engineering methods of capturing CO₂ from the air and its utilization. Sustainable Production and Consumption, 37, 78-95. https://doi.org/10.1016/j.spc.2023.02.011.

Hernández, B, Martín, M. (2016). Optimal composition of the biogas for dry reforming in the Production of Methanol Ind. Eng. Chem. Res. 55(23), 6677–6685.

Janković, T., Straathof, A. J. J., Kiss, A. A. (2024). Enhanced isobutanol recovery from fermentation broth for sustainable biofuels production. Energy Conversion and Management: X, 21, 100520. https://doi.org/10.1016/j.ecmx.2023.100520.

Martín M., Grossmann I. E. (2017). Towards zero CO₂ emissions in the production of Methanol from switchgrass. CO₂ to methanol. Comp. Chem. Eng. 105, 308–316. https://doi.org/10.1016/j.compchemeng.2016.11.030

Martín, M., Grossmann, I. E. (2012). Energy optimization of bioethanol production via hydrolysis of switchgrass. AIChE Journal, 58(5), 1538-1549. https://doi.org/10.1002/aic.12735.

Martín, M., Grossmann, I.E. (2011). Energy optimization of bioethanol production via gasification of switchgrass. AIChE J. 57(12), 3408-3428. https://doi.org/10.1002/aic.12544.

Su, Y., Zhang, W., Zhang, A., Shao, W. (2020). Biorefinery: The Production of Isobutanol from Biomass Feedstocks. Applied Sciences, 10(22), 8222. https://doi.org/10.3390/app10228222.

Wingad, R. L., Bergström, E. J. E., Everett, M., Pellow, K. J., Wass, D. F. (2016). Catalytic conversion of methanol/ethanol to isobutanol – a highly selective route to an advanced biofuel. Chemical Communications, 52(36), 5202-5204. https://doi.org/10.1039/C6CC01599A.